1. Field of the Invention
This invention relates to a pneumatic radial tire, and more particularly to a heavy duty radial tire having an aspect ratio of no more than 60% and largely improving the durability of the bead portion.
2. Description of Related Art
In the pneumatic radial tire running under loading, a pair of sidewall portions located in correspondence to a ground contact face are largely bent, while a bead portion located outward from a rim flange in a radial direction of the tire is deformed toward the outside of the tire or so-called deformation phenomenon of the bead portion is caused, whereby a large compression strain is applied to a turn-up end portion of a carcass ply existing in the bead portion or the sidewall portion.
As a countermeasure for such a compression strain, JP-A-5-16618, JP-A-8-324214 and the like propose an improvement in the structure of a stiffener composed of a soft rubber stock and a hard rubber stock, and an improvement of a cushion rubber arranged at an outside of a turn-up portion of a carcass ply in the widthwise direction of the tire, whereby the improvement of the durability of the bead portion is realized.
On the other hand, the deformation in the circumferential direction is caused at zones ranging from the bead portion to the sidewall portion located in correspondence to a stepping-in part and a kicking-out part of the tread portion in the ground contact face of the tire, respectively, whereby a shearing strain in the circumferential direction is created in the turn-up portion of the carcass ply.
Such a shearing strain can be mitigated by selecting an arranging conformation of the hard rubber stock in the stiffener as disclosed in JP-A-8-225005, whereby the durability of the bead portion can be enhanced.
Recently, the lowering of the aspect ratio of the tire, the increase of the tire size and the like are advanced even in tires for heavy duty vehicles such as trucks, buses and the like and also, the number of tire recappings is increased to increase total input to the bead portion accompanied with the running of the tire under loading. For this end, the durability of the bead portion tends to be lacking even in the aforementioned countermeasures. This is particularly serious in the large-size tires having an aspect ratio of no more than 60%.
That is, the above problem is considered due to the fact that the shearing strain in the circumferential direction is increased in the vicinity of the turn-up end of the carcass ply because the length of a zone ranging from a portion of a maximum tire width to a buttress portion in the cross section, which effectively contributes to absorb the deformation in the circumferential direction of the tire, is decreased accompanied with the lowering of the aspect ratio of the tire, and also the absolute amount of deformation in the circumferential direction in the traction and braking or the shearing strain in the circumferential direction is increased under an advancement in performances of the vehicle and the tire, the increase of the tire size and the like.
It is, therefore, an object of the invention to provide a pneumatic radial tire largely improving the durability of the bead portion by decreasing the shearing strain in the circumferential direction in the vicinity of the turn-up portion of the carcass ply.
According to the invention, there is the provision of in a pneumatic radial tire having an aspect ratio of no more than 60% and comprising at least one carcass ply composed of a main body portion extending between a pair of bead cores each embedded in the respective bead portion and a turn-up portion wound outward around each bead core in a radial direction of the tire, a stiffener arranged between the main body portion and the turn-up portion on an outside of the bead core in the radial direction of the tire, extending outward in the radial direction of the tire, and made of a soft rubber stock and a hard rubber stock, and a reinforcing member extending at an outside of the turn-up portion in a widthwise direction of the tire through a cushion rubber, the improvement wherein when the tire is inflated under an air pressure corresponding to 10% of a maximum air pressure, a height of an outer end of the turn-up portion in the radial direction is no more than 0.33 times a section height of the carcass ply, and a ratio of a thickness between the main body portion and the reinforcing member to a thickness between the main body portion and the outer end of the turn-up portion in the radial direction as measured on a normal line drawn from the outer end of the turn-up portion in the radial direction onto an outer surface of the main body portion is within a range of 1.2-1.8, preferably 1.3-1.7.
Moreover, the term xe2x80x9csection height of the carcass plyxe2x80x9d used herein means a height when measuring along the radial direction of the tire between an innermost position of the carcass in the radial direction and an outermost position thereof.
And also, the term xe2x80x9cmaximum air pressurexe2x80x9d used herein means an air pressure corresponding to a maximum load capacity defined in JATMA YEAR BOOK. When the tire is inflated under the air pressure, the tire is first mounted onto an approved rim defined in JATMA YEAR BOOK.
Although the tire having the aspect ratio of no more than 60% is particularly serious in the lacking of the durability of the bead portion, according to the invention, the height of the outer end of the turn-up portion of the carcass ply in the radial direction is made no more than 0.33 times the section height of the carcass ply, whereby force suppressing the deformation of the tire through the rim flange is sufficiently applied to a portion located in the vicinity of the outer end of the turn-up portion in the radial direction under an action of the bead core at a state of mounting the tire onto the rim to advantageously decrease the deformation in the circumferential direction of the portion located in the vicinity of the outer end of the turn-up portion in the radial direction and hence the shearing strain in the circumferential direction, so that the separation of the outer end of the turn-up portion in the radial direction can effectively be prevented. In other words, when the height of the outer end of the turn-up portion in the radial direction is more than 0.33 times, the outer end of the turn-up portion can not sufficiently be subjected to the force suppressing the deformation of the tire through the rim flange.
And also, according to the invention, the ratio of the thickness between the main body portion and the reinforcing member to the thickness between the main body portion and the outer end of the turn-up portion in the radial direction is made 1.2-1.8, more preferably, 1.3-1.7, whereby the reinforcing member is arranged so as to sufficiently be distant outward from the outer end in the radial direction to the outer portion in the widthwise direction of the tire in which the torsion deformation at a ground contact area is large under loading to thereby largely improve the reinforcing effect by the reinforcing member, whereby the amount of deformation in the circumferential direction can advantageously be decreased. That is, the reinforcing member more reinforces the bead portion to the deformation in the circumferential direction under the decrease of the absorption area thereof resulted from the lowering of the aspect ratio of the tire, so that the shearing strain in the circumferential direction in the vicinity of the outer end of the turn-up portion can effectively be decreased.
Therefore, when the ratio is less than 1.2, it is difficult to sufficiently develop the reinforcing function through the reinforcing member, while when it exceeds 1.8, the quantity of heat generation in the cushion rubber between the turn-up portion and the reinforcing member is increased to prematurely promote the thermal deterioration due to the rise of temperature during the running and hence the durability is not totally improved.
In a preferable embodiment of the invention, a wire chafer is arranged from the main body portion of the carcass ply to the turn-up portion thereof along the carcass ply and an outer end thereof in the radial direction along the main body portion is positioned outward from the outer end of the turn-up portion of the carcass ply in the radial direction.
In this case, when cords of the wire chafer are crossed with cords of the carcass ply, the deformation of the bead portion in the circumferential direction can effectively be controlled by the wire chafer.
Moreover, if the outer end of the turn-up portion in the radial direction is positioned outward from the outer end of the wire chafer in the radial direction, the deformation in the circumferential direction concentrates in a zone located in the vicinity of the outer end of the turn-up portion, and hence there is caused a problem that the increase of the shearing strain at the zone become unavoidable.
In another preferable embodiment of the invention, the reinforcing member located at the outside of the turn-up portion in the widthwise direction of the tire is constituted with three or four nylon chafers, at least one of which chafers extends up to an inner portion of the bead core just beneath the bead core in the radial direction, and outer ends of these nylon chafers in the radial direction are positioned outward from the outer end of the turn-up portion in the radial direction.
In this case, the reinforcing member arranged at a large deformation region is made of nylon cords and the cord itself is flexible, so that the concentration of strain at the end of the reinforcing member can be controlled as compared with a case using steel cords to avoid the separation failure at the end of the reinforcing member.
And also, the reinforcing member is comprised of three or four nylon chafers, so that the required reinforcing function can effectively be developed. In this case, when the positions of the outer ends of these nylon chafers in the radial direction are irregular, the shearing strain in the circumferential direction produced in the vicinity of the ends of the nylon chafers can advantageously be dispersed.
In addition, at least one nylon chafer is extended up to the inner portion of the bead core just beneath the bead core in the radial direction to spread the function of the bead core fixing the bead portion to the nylon chafer, whereby the shearing strain in the circumferential direction can forcedly be decreased.
When the number of the nylon chafers is not more than two, the required reinforcing function can not sufficiently be developed, while when it is five or more, rubber is early thermal-degraded by the heat generation accompanied with the increase of the thickness and it is difficult to expect the improvement of the durability of the bead portion.
In the other preferable embodiment of the invention, the stiffener is constituted with a hard rubber stock located inward in the radial direction of the tire and having a Shore-A hardness of no less than 70, and a soft rubber stock located outward in the radial direction of the tire and having a Shore-A hardness of no more than 65. In this case, the deformation (bending deformation, deformation in the circumferential direction) during the running and the lose of bead shape produced in a long-term use can be controlled by the hard rubber stock to improve the durability. However, if the volume of the hard rubber stock is increased over a required volume, the flexible zone is decreased, and the deformation controlling effect itself is weakened, and also the hard rubber stock usually indicating a large heat generation is arranged at the large deformation region to cause demerits in view of the rise of temperature. Furthermore, when the turn-up portion of the carcass ply contacts with the hard rubber stock, strain is inversely concentrated in a contact point by a large difference of rigidity therebetween and hence the improvement of the durability can not be attained. Therefore, the stiffener is preferable to be constituted with the soft rubber stock and the hard rubber stock.
In a yet further preferable embodiment of the invention, a hardness of the cushion rubber is no more than 58, preferably 32-58 as a Shore-A hardness. The cushion rubber acts as a layer of mitigating the shearing strain in the circumferential direction between the turn-up portion and the reinforcing member. When the hardness exceeds 58, the action mitigating the strain based on the deformation of the cushion rubber itself is not sufficient. Moreover, when the hardness is less than 32, the amount of deformation of the cushion rubber in the circumferential direction is too large, so that there is caused a fear of creating not only the separation failure of the outer end of the turn-up portion in the radial direction from the cushion rubber but also the troubles in the cushion rubber.
In a still further preferable embodiment of the invention, a thickness of the hard rubber stock on the normal line drawn from the outer end of the turn-up portion in the radial direction toward the outer surface of the main body portion is made no more than 0.5 times the thickness between the outer end of the turn-up portion in the radial direction and the main body portion. In this case, the difference of rigidity between the inside and the outside of the bead portion in the radial direction of the tire can advantageously be removed while effectively preventing the concentration of the compression strain to the inside of the turn-up end in the widthwise direction of the tire. That is, when the above thickness exceeds 0.5 times, there is caused a fear of separating the outer end of the turn-up portion due to the concentration of the compression strain in the zone located in the vicinity of the outer end.